2. On June 28, 1914, the Archduke Francis Ferdinand, who would have been the next king of Austria-Hungary, was shot and killed (along with his wife Sophie) in Sarajevo, a city in Bosnia. The assassin had been a Serbian nationalist. Hungary asked Germany for help and Serbia looked to Russia. Austria-Hungary then declared war on Serbia. More Information can be found here.

3. Exactly 1 month after the assasination (June 28, 1914) Austria-Hungary declared war on Serbia (The assassin had been a Serbian nationalist.) Previously, Russia had made allies with Serbia, and Germany had become allies with Austria-Hungary. Four days after war was declared, Germany officially began war with Russia. This marked the beginning of WW1.